The present invention relates to non-aqueous electrolyte secondary batteries. More specifically, the present invention relates to a non-aqueous electrolyte secondary battery including an improved negative electrode, and having high capacity, long life and excellent high-rate discharge characteristics.
In the positive electrode for a non-aqueous electrolyte secondary battery, chalcogenides such as LiMn2O4, LiCoO2, LiNiO2, V2O5, Cr2O5, MnO2, TiS2 and MoS2 may be employed. They have a layered or tunneled structure permitting the intercalation and deintercalation of lithium ion.
On the other hand, metallic lithium may be employed in the negative electrode for a non-aqueous electrolyte secondary battery. In this case, a battery of high energy density and high voltage can be realized. However, the use of metallic lithium results in lithium dendrites deposited on the surface of the metallic lithium during charging, which may reduce the charge and discharge efficiency, or cause an internal short-circuit due to the contact between the dendrites and the positive electrode. Therefore, graphite-based carbon materials which have lower capacity than metallic lithium but are capable of reversibly absorbing and desorbing lithium, have recently been employed in the negative electrode. Then, lithium ion secondary batteries which are excellent in cycle characteristics and safety have been put into practical uses.
However, the practical capacity of the carbon material is as small as 350 mAh/g, and the theoretical density of carbon material is as low as 2.2 g/cc. Therefore, alloy particles capable of yielding a negative electrode having higher capacity are desired to be used as the negative electrode material.
However, an alloy is generally pulverized by its repeated expansion and contraction due to the intercalation and deintercalation of lithium ion. The pulverized alloy loses the contact with other alloy particles, conductive agent and the like in the negative electrode, reducing the conductivity of the negative electrode. In other words, the pulverized alloy appears to be an inactive material, so that the capacity of the battery is reduced.
In view of the above-described problems, the following proposals have been made.
Japanese Unexamined Patent Publication No. hei 11-86854 proposes to allow a phase capable of absorbing lithium and a phase incapable of absorbing the same to coexist in an alloy particle that is the negative electrode material. In a charged state, the phase incapable of absorbing lithium relaxes the expansion stress of the phase capable of absorbing lithium, so that the pulverization due to the expansion of the particle can be suppressed.
Japanese Unexamined Patent Publication No. hei 11-86853 proposes to allow two or more types of phases each capable of absorbing lithium to coexist in an alloy particle that is the negative electrode material. Although the both phases expand at the time of lithium absorption, the expansion stress of the respective phases are different from each other; therefore, the pulverization of the particle can be suppressed by narrowing the difference of the expansion coefficient between the two phases. The respective phases are present as fine crystal grains within the particle. It is considered that the stress is dispersed on the interfaces between the crystal grains at the time of lithium absorption.
However, even a negative electrode material in line with the above proposals is not sufficient to suppress the pulverization and to prevent the degradation of the cycle characteristics due to repeated charging and discharging. The possible reasons are as follows.
According to the former proposal, the expansion coefficients of the respective phases are significantly different from each other. Therefore, uneven stress is produced in the particle, causing a strong partial stress. The phase incapable of absorbing lithium is not able to sufficiently relax this stress. For this reason, it is considered that the particle is eventually pulverized and separated from the conductive network. In addition, the phase incapable of absorbing lithium prevents the movement of lithium ion, so that the high-rate discharge characteristics of the battery are not sufficient.
According to the latter proposal, the both phases absorb lithium ion and thus the difference of the expansion coefficient between the phases is small at the time of lithium absorption. However, the volume expansion of the entire particle is large and, therefore, it is likely that a gap is formed between particles or between a particle and a current collector thereby to deteriorate the current collection. For this reason, it is considered that alloy particles are more likely to be separated from the conductive network in the negative electrode. In other words, according to the latter proposal, it is difficult to suppress a reduction in discharge capacity during charge/discharge cycles, although it is possible to suppress the pulverization of the alloy particles due to the expansion and contraction.
On the other hand, there is also a proposal to employ metal oxides as the negative electrode material.
For example, Idota et al. proposes a negative electrode containing a tin oxide and having high capacity as well as long life (Science, 276, 5317, 1395-1397 (1997)). However, in the process of the initial charging, it is necessary to reduce bivalent or tetravalent tin in the tin oxide to a metallic state of zero valent. Accordingly, the irreversible capacity of the battery increases, resulting in an increased amount of lithium which does not participate in charge/discharge cycles. Therefore, only those batteries having low capacities can be obtained.
As described above, batteries using negative electrodes comprising conventional alloys and oxides have the problem that they are easily degraded in cycle characteristics and tend to have a decreased capacity.
It is an object of the present invention to provide a non-aqueous electrolyte secondary battery having high capacity, long cycle life and excellent high-rate discharge characteristics.
The present invention relates to a non-aqueous electrolyte secondary battery comprising: a positive electrode capable of absorbing and desorbing lithium ion; a negative electrode capable of absorbing and desorbing lithium ion; and a non-aqueous electrolyte containing a lithium salt, the negative electrode comprising: an alloy particle containing at least two selected from the group consisting of metal elements and semimetal elements; and one selected from the group consisting of oxygen and nitrogen, wherein the alloy particle has a phase A capable of electrochemically absorbing and desorbing lithium ion and a phase B which is incapable of electrochemically absorbing and desorbing lithium ion and has lithium ion conductivity or lithium ion permeability and wherein the total of an oxygen content Wao and a nitrogen content Wan is less than 0.5 wt % in the phase A and the total of an oxygen content Wbo and a nitrogen content Wbn is not less than 1.0 wt % in the phase B. As the semimetals, Si, Sb and the like may be used.
It is preferred that the phase A contain at least one selected from the group consisting of Sn, Si, Al, Ga, In, Pb, Sb and Bi.
It is preferred that the phase B contain at least one selected from the group consisting of Ti, Zr and rare earth elements.
It is preferred that the phase A be surrounded by the phase B.
It is preferred that the oxygen content Wao and the nitrogen content Wan in the phase A and the oxygen content Wbo and the nitrogen content Wbn in the phase B satisfy {(Wbo+Wbn)/(Wao+Wan))} greater than 4.
It is preferred that an oxygen content Wo and a nitrogen content Wn in the alloy particles satisfy 0 less than Wo less than 10 wt %, 0 less than Wn less than 10 wt % and 0.5 wt %xe2x89xa6Wo+Wnxe2x89xa610 wt %.
The alloy particles may further contain at least one selected from the group consisting of fluorine, sulfur and phosphorous. In this case, it is preferred that the total of a content Wf of at least one selected from the group consisting of fluorine, sulfur and phosphorous, the oxygen content Wo and the nitrogen content Wn be 0.5 to 10 wt % in the alloy particles. Additionally, it is preferred that the Wf be 0.5 to 1 wt %.